The refractories currently used as interior linings of combustion chambers are prevalently mullite based. They are aluminosilicate refractories characterized by a good shock resistance.
However, in the presence of biomass, the combustion releases alkaline species, such as potassium (K) and sodium (Na), which act upon refractories according to two prevalent pathways:                1. they react with the silica (SiO2), lowering its melting point and thus facilitating phenomena of corrosion/erosion;        2. in the absence of silica, they react with the alumina (Al2O3), producing crystalline phases with an expansion in volume and possible chipping of the refractory.        
In the particular case of plants that use fluidized bed combustion, the phenomenon of corrosion is accompanied by that of erosion, caused by the circulation of the bed. Refractories containing silica are particularly damaged in the presence of alkali corrosion and erosion caused by the fluidized bed.
To obviate such phenomena of damage, two types of refractories are used:
1. Dense materials with a high content of alumina;
2. Dense materials based on silicon carbide (SiC).
These two solutions are exceedingly costly and introduce problems of thermal cycling.
An alternative proposal is a refractory with a composition gradient (compositionally graded refractory) which optimizes the property of shock resistance together with corrosion resistance.
For the design of blast furnace hearths, patent CN203728861 proposes a distribution of refractory carbon bricks with a thermal conductivity gradient in order to reduce breakage caused by thermal shocks; however, it provides no indications as to how also to solve the problem of corrosion resistance.